/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/MoviePipelineQueueEngineSubsystem.h
Returns true if there is an active executor working on producing a movie. This could be actively rendering frames, or working on post processing (finalizing file writes, etc.). UseGetActiveExecutor()and query it directly for more information, progress updates, etc.
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintPure, Category="Movie Render Pipeline|Rendering") bool IsRendering() const